The present invention relates to disc brakes for vehicles, and in particular to brake pads in disc brakes, such as air-operated disc brakes utilized on commercial vehicles.
An example of a commercial vehicle air-operated disc brake is shown in FIG. 1. In the FIG. 1 embodiment the disc brake 1 includes a brake disk 2 which rotates in direction A about its rotation axis B. A brake caliper 3 straddling the brake disk 2 is affixed to a carrier mount 4 which in turn is fixed to a vehicle axle, typically via a torque plate or a brake spider (not illustrated). In this embodiment the carrier mount 4 receives and supports both the caliper 3 and the brake pads 6. The caliper 3 is actuated in this embodiment by a pneumatic actuator (not illustrated) mounted at the actuator mounting face 5 of the caliper. The actuator acts upon a brake pad application mechanism contained within caliper 3 to press the brake pads 6 against the brake disk 2 to slow the vehicle. The present invention is not restricted to a particular type of brake actuator, for example, a pneumatic actuator or an electrically-driven actuator may be used. Nor is the invention limited to a particular type of brake caliper mount arrangement. For example, the brake caliper may be mounted in a fixed manner on a carrier or may be a sliding caliper.
In disc brake applications such as commercial vehicle disc brakes the brake pads typically have had a generally rectangular shape, in part due to the limitations on the size and configuration of the disc brake components (the disc brake having to exist within a highly-space constrained envelope provided by wheel rims), and in part due to cost and structural limitations discussed further below. An example of such a previous brake pad is shown in FIG. 4.
A common feature of a previous brake pad 20 is their having essentially parallel lateral sides 22, 23, i.e., the brake pad sides facing in the circumferential direction of the brake disc toward adjacent brake pad abutment surfaces are parallel to one another. The generally rectangular shape may include radially inner and radially outer sides of the brake pad 24, 25 that are slightly curved to generally follow the curvature of the brake disc as shown in FIG. 4, or in the case of the radially outer side, follow the shape of an adjacent outer region of the brake caliper (not illustrated). The use of parallel lateral sides of the brake pad has in part been the de facto standard in commercial vehicle disc brakes in part due to practical manufacturing considerations (for example, less costly machining of brake pad abutment surfaces and parallel-sided brake pad backing plates) and in part due to structural reasons to ensure adequate brake pad abutment strength, wear, and braking force absorption performance.
With their generally rectangular shape, the previous commercial vehicle disc brake pads have presented to the brake disc essentially constant width and height profiles from one lateral side of the brake pad to the other. Such brake pad shapes have several disadvantages during brake operation. Among these is the fact that the specific braking energy transfer from the brake disc to the brake pad is not constant across the radial height of the brake pad. Instead, the energy transfer varies as a function of radial height relative to the rotation axis of the brake disc (i.e., braking torque varying as a function of the distance from the brake disc rotation axis, where force×distance=torque), and as a function of the length of the friction surface of the brake pad friction material at different radial heights. As a result, the energy transfer to the brake pad, and the resulting localized wear of the brake pad, is inconsistent across the face of the brake pad friction material. This can lead to premature wear of the friction material in some areas of the brake pad and thereby shorten the time before the brake pad must be replaced.
The present invention addresses this and other problems by providing a brake pad with more efficient and even braking energy transfer distribution across the face of the brake pad lining material. The approach of the present invention provides for more even pad lining material wear, thereby extending service life of the brake pad. The improved brake pad performance also enables reduction in overall brake size by allowing the use of smaller brake pads while still providing satisfactory braking performance.
In an embodiment of the present invention, the brake pad lining material, and preferably the brake pad backing plate carrying the lining material, has a generally arc-shaped profile, with the radially outer portion of the lining material having a width in the circumferential direction that is longer than the width of the lining material at the radially inner portion of the brake pad. Preferably, the width of the brake pad lining material as a function of radial distance from the brake disc rotation axis is established by generally aligning the lateral sides of the lining material along radial lines that intersect at or near the rotation axis of the brake disc. The lateral sides of the brake pad need not be exactly aligned with the radial lines from the rotation axis; rather the present invention contemplates the greatest lining material width at the radially outer region of the brake pad, while the width is smaller at the radially inner region of the lining material. The closer the intersection is to the center of the brake disc rotor, the more efficient the energy distribution at the pad-disc interface.
Another further advantage of the present invention is that the reduced width in the radially inner region of the brake pad permits the abutment faces of the brake pad carrier and the lateral sides of the brake pad to meet along a line that is more nearly perpendicular to a radius from the rotation axis. This arrangement allows the transfer braking forces between the lateral side of the brake pad and pad abutment surface of the pad carrier at or nearly normal to the abutment line. This provides for more uniform distribution of the abutment forces over the abutment surface, i.e., more even (and thus lower) contact pressures, helping minimize brake pad vibrations and associated brake noise, improved fatigue life performance and reduce component wear.
In addition, the arrangements can help in reducing the effects of “pad kick,” an in-place rotation of the brake pad that can generate undesired brake application noise due to pad vibrations, increase fatigue damage to typical brake pad retaining hardware (e.g., over-pad leaf springs) and increase wear and damage to the brake pad and/or brake caliper mounting structure. An illustration of pad kick is provided in FIG. 4. When a brake pad 101 is applied against a friction surface of a brake disk (not illustrated) which is rotating in direction DR, the brake disk's rotation induces motion and reaction forces between the brake pad 101 and its adjacent mount abutment surfaces (not illustrated). At the leading edge 102 of the brake pad the brake pad attempts to move upward in direction LU in response to the friction forces along the face of the brake pad (illustrated here by force arrows across the face of brake pad 101). At the trailing edge 103 of the brake pad, the brake pad attempts to move downward in direction TD. However, because the brake pad 101 is constrained by adjacent mount abutment surfaces, the overall motion of the brake pad is generally a rotation about an axis parallel to the brake disk rotation axis. This motion may be unilateral during the brake application, or may manifest itself as a moderate-to-severe oscillation of the brake pad in its mount, significantly increasing wear of the abutting brake pad and mount surfaces.
One of ordinary skill in the art will recognize that the brake pad support function may be provided by a brake caliper mount designed to support the brake pads, or by a brake pad carrier which is separate from the caliper mounting structure. For convenience in this description, the terms caliper carrier, caliper mount and brake pad carrier may be interchanged without intending to limit the brake par supporting structure to any specific brake pad and brake caliper carrying structure.
A further advantage of the present invention is that the reduced width in the radially inner region of the brake pad permits brake pad retention features, such as those disclosed in co-pending application Ser. No. 14/640,152, to be moved closer together to enable further reduction in the size of the disc brake components while maintaining a desired level of braking performance and/or or increasing braking performance by increasing brake pad lining surface area while still keeping overall brake size within the space-constrained envelope of the wheel rim and other nearby components.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.